test/ebnf.py: `\~` is now substituted by mangled whitespace in regular expressions for _skip and _resume

- DHParser/templates/tst_DSL_grammar.pyi: grammar-test directory name changed from "grammar_test" to "test_grammar"

- DHParser/test_preprocess.py: fixed test-case that relies on at least on a "max_parser_dropouts" config value of at least 2

- DHParser/ebnf.py: semantics for string-arguments of _skip and _lookahead directive. Will automatically be considered as lookahead

- DHParser/parse.py: resume and skip directives now jump to the reentry-point right after the match of the regular expression for re-entry